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Date: Wed, 07 Apr 2021 16:38:46 -0400
From: "Valdis Klētnieks" <>
To: John Wood <>
Cc:, Andi Kleen <>,
    Kees Cook <>,
Subject: Re: Notify special task kill using wait* functions

On Wed, 07 Apr 2021 19:51:51 +0200, John Wood said:

> When brute detects a brute force attack through the fork system call
> (killing p3) it will mark the binary file executed by p3 as "not allowed".
> From now on, any execve that try to run this binary will fail. This way it
> is not necessary to notify nothing to userspace and also we avoid an exec
> brute force attack due to the respawn of processes [2] by a supervisor
> (abused or not by a bad guy).

You're not thinking evil enough. :)

I didn't even finish the line that starts "From now on.." before I started
wondering "How can I abuse this to hang or crash a system?"  And it only took
me a few seconds to come up with an attack. All you need to do is find a way to
sigsegv /bin/bash... and that's easy to do by forking, excecve /bin/bash, and
then use ptrace() to screw the child process's stack and cause a sigsegv.

Say goodnight Gracie...

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